1. It would make more sense for shamans to wear leather. Most of iconic shaman heroes, such as World Shaman Thrall, Drek'Thar, OG Rehgar and Muln, all wear leather armor. Real life shamans and shamans in most other fictional fantasy worlds also wear leather.
2. It would make more sense for hunters to wear leather. Some of iconic hunter heroes, such as OG Shandris and Rexxar, wear leather armor. You could argue that Sylvanas wears mail but she is more of a Dark Ranger or a military ranger than a hunter. It makes no sense for a hunter to stomp through the forest in clunky, noisy mail.
3. Mail, as it is designed by Blizzard, has no identity. Some mail sets are clearly leather (e.g. Gronnstalker gear). Other sets are clearly undercooked plate. There are very few sets that look like actual mail and for a good reason. Real life mail is boring as hell. It's like a piece of fabric made of linking metal.
4. It's a waste of resources. Mail is only represented by 2 classes (shamans and hunters) while Plate has 3, leather had 4!!! and cloth has another 3.
To me mail feels like a relic from vanilla WoW where Blizzard arbitrarily decided that mail classes should upgrade to plate and leather to mail at some level. For the reasons outlined above I believe that Blizzard should either move both hunters and shamans to leather armor or lift transmog restrictions and allow people to mog things one armor class below (plate to mail, mail to leather, leather to cloth).